In JP-8-133016A, an engine start system (also called as an immobilizer) for a vehicle is described which controls an engine of the vehicle based on result of comparison between two IDs, one of which is an authentication ID stored in a wireless mobile communicator (which also serves as a vehicle entrance key) and the other one of which is a master ID stored in a device in the vehicle. The engine start system includes an engine control unit for controlling the engine and an immobilizing control unit for controlling the engine control unit based on the result of the comparison. The immobilizing control unit allows the engine control unit to start the engine when the two IDs are identical, that is, when an authentication of the authentication ID is successful. In contrast, the immobilizing control unit prohibits starting the engine when the two IDs are not identical, that is, when the authentication of the authentication ID fails. Thus, the engine start system rejects an improper request for starting the engine which is made by an unauthorized person.
Some criminal acts may be made which compromises the engine start system by replacing the immobilizing control unit with another control unit. As a measure against the criminal acts, an advanced engine start system is provided in which a controller is connected through an on-board communication network with on board electrical device. In the advanced engine start system, a device ID is stored in each of on-board electrical devices having a risk of replacement. In addition, master device IDs for comparison with the respective device IDs are in the controller. The advanced engine start system performs a device individual authentication function (also referred to as multi-device authentication function) in which the controller makes authentications between the device IDs and the respective master device IDs by means of communication through the on-board communication network. The controller allows the engine to start only when the authentications of the device IDs are successful, that is, the electrical devices are legitimate.
The device individual authentication function can be applied to not only to the immobilizing control unit but also security devices (for example, a door lock controller) which do not have close relation with a starting operation of the engine. The device individual authentication function is applied to electrical devices (for example, a vehicle air conditioning system, a vehicle audio system or a vehicle navigation system) which have no relation with the starting operation of the engine, in order to prevent replacing a legitimate electrical device with a not-legitimate electrical device.
In the device individual authentication function, the engine is prohibited from starting when at least one of the authentications of the device IDs fails. In addition, each of the authentications of the device IDs fails not only when a legitimate device is replaced with another device but also when a trouble occurs in which the legitimate device cannot output its device ID because of, for example, malfunction. The engine is therefore prohibited from starting even when an electrical device which has no relation with the engine is malfunctioning. Such a problem puts a user of the vehicle in a serious situation in which the user cannot move the vehicle to a repair plant by himself/herself and is therefore obliged to have the vehicle towed. In addition, the user may not be able to turn on an air conditioning system of the vehicle. The conventional engine start controlling system does not take any measure against the problem.